The present invention relates to the measurement of line earth loop impedance in circuits protected by Residual Current Operated Circuit Breakers (RCCBs).
Measurement of the phase—earth loop impedance in an AC power supply is mandatory in many countries (for example, Great Britain). A simple test (with no RCCB present) would involve measuring the voltage between the phase and earth lines as a test current is passed with and without a test resistor of a known value in the circuit. Since loop impedances are typically of the order of 0.5 Ohms a fairly large test current (for example, 20 A) is required if the voltage to measure is to be discernable in the presence of large line voltages (for example, 230 volts), and tens of volts random noise due to loads and generators being switched on the public supply network.
Modern circuits are commonly protected by RCCBs. These detect any imbalance in the current flow in the phase neutral loop, and at relatively low trip levels (around 30 mA) will break the circuit to prevent any danger of personal injury. The simple method outlined above would trip an RCCB since the test current required is higher than the trip value. As indicated above, using a test current with a magnitude less than this trip voltage produces a test voltage that is extremely difficult to measure against the background noise. For instance, a test current of 10 mA and a loop impedance of 0.5 Ohms would produce a voltage result of only 5 millivolts.
For this reason, previous methods of overcoming this difficulty have often relied upon deliberately circumventing the RCCB's current detection facility, thus effectively deceiving the RCCB into accepting a current higher than the trip value. For example, an early weakness of RCCB design was exploited by the device described in European patent EP0295800, which employs a test current gently ramped up from a value below the trip level. However, this technique is ineffective in the context of many modern RCCB types. In general, the design of the RCCB employed must be known for such techniques to find utility. This information is not typically known to the tester.
RCCBs themselves have developed from tripping on AC only (Type AC) to tripping on AC and pulsed DC (Type A) and now to tripping on AC, pulsed DC and also smooth DC. This is a continual process of improving the tripping characteristics of RCCBs to cover any type of leakage current imaginable. It becomes more necessary than ever to use a technique that does not rely on deceiving the RCCB into accepting a test current higher than its rated trip level.
Some such techniques have been suggested but typically provide results that lack sufficient repeatability. An example is described in European patent EP0706663, where the test result is extracted using a switched capacitor n-path filter technique to remove the interference caused by the supply signal.